fix(schema): cascade-delete VideoConfig/LintResult/BulkJobItem/VideoPlaylist with their Video

Channel import crashed on the hard-delete purge path (videos confirmed
gone from YouTube for 30+ days): prisma.video.deleteMany() hit a FK
violation on VideoConfig_videoId_fkey. All four required relations
pointing at Video had no onDelete behavior set, defaulting to RESTRICT
- any of the four would have hit the same crash, not just VideoConfig.
First real channel import against a real, aged dataset is what
surfaced this; nothing had exercised the 30-day hard-delete path before.

backend/prisma/schema.prisma

@@ -174,7 +174,7 @@ enum LintStatus {
 model VideoConfig {
   id              String    @id @default(cuid())
   videoId         String    @unique
-  video           Video     @relation(fields: [videoId], references: [id])
+  video           Video     @relation(fields: [videoId], references: [id], onDelete: Cascade)
   templateId      String?
   blockOrder      Json
   blockOverrides  Json
@@ -312,7 +312,7 @@ model SavedView {
 model LintResult {
   id            String       @id @default(cuid())
   videoId       String
-  video         Video        @relation(fields: [videoId], references: [id])
+  video         Video        @relation(fields: [videoId], references: [id], onDelete: Cascade)
   ruleCode      String
   severity      LintSeverity
   targetField   String?
@@ -369,7 +369,7 @@ model BulkJobItem {
   bulkJobId      String
   bulkJob        BulkJob  @relation(fields: [bulkJobId], references: [id])
   videoId        String
-  video          Video    @relation(fields: [videoId], references: [id])
+  video          Video    @relation(fields: [videoId], references: [id], onDelete: Cascade)
   beforeSnapshot Json?
   afterSnapshot  Json?
   status         String   @default("pending")
@@ -415,7 +415,7 @@ model VideoPlaylist {
   videoId    String
   playlistId String
   position   Int?
-  video      Video    @relation(fields: [videoId], references: [id])
+  video      Video    @relation(fields: [videoId], references: [id], onDelete: Cascade)
   playlist   Playlist @relation(fields: [playlistId], references: [id])
 
   @@id([videoId, playlistId])